For the next installment in “The Legend of Zelda” series, “Link’s Awakening”, Koizumi was once again tasked with designing the game’s manual. Koizumi’s involvement in the project was much earlier than when he was assigned to create the manual for “A Link to the Past”, this time prior to an in-game story being created. As a result, Koizumi’s ideas were incorporated into the game and he wounded up creating the entire story, influencing the outcome of the game.

His involvement in the creation of the story for “Link’s Awakening” led him to get involved in other areas of game creation other than just the creation of manuals and script creation. During the creation of the game he assisted with the creation of event design for the interaction with the villagers and designed the bosses’ behavioral patterns. This gave Koizumi a broader background in game creation leading to future opportunities within Nintendo, such as working as a Computer Graphics (CG) Designer on Super Mario World 2: Yoshi’s Island before moving on to working on three dimensional games with the release of the Nintendo 64.

While Nintendo was in the process of coming up with concept ideas for Super Mario 64, Koizumi took the opportunity to create some game design concepts using an Amiga computer and showed them to Shigeru Miyamoto. Miyamoto was impressed with the designs and brought Koizumi onto the project as assistant director along with Takashi Tezuka. Koizumi created the first 3D model of Mario for the game and started working very closely with Miyamoto on gameplay elements such as character movement and camera positioning. Three dimensional games were virtually unexplored at the time and the team had to come up with solutions for many new elements that did not exist in two dimensional games. Many of the gameplay mechanics they created set the standard for 3D games and were utilized across the industry for decades.

The confidence that Miyamoto and Nintendo had in Koizumi after his contribution in the creation of Super Mario 64 landed him a Director position in the first 3D Legend of Zelda project. He was assigned as the 3D System Director for “The Legend of Zelda: Ocarina of Time” and also contributed with character design. Multiple directors were assigned to the large-scale project such as Toru Osawa, Yoichi Yamada and Eiji Aonuma. Koizumi’s role consisted of environment construction, camera design, creating the 3D models of Link (both child and adult version), Epona and Navi. He was also involved in the creation of the Z-Targeting system, which would become a standard in 3D action games.

After the release of “Ocarina of Time” Koizumi along with Aonuma where assigned as Game System Directors for “The Legend of Zelda: Majora’s Mask”. Koizumi came up with the scenario of Termina being threatened by the falling Moon, the stories of the inhabitants of Clock Town and came up with the various transformations of Link via masks. Many of the serious elements in Majora’s Mask came from Koizumi and remain uniquely distinctive in the franchise.

After the completion of “Majora’s Mask” Nintendo decided to entrust Eiji Aonuma to lead the direction of “The Legend of Zelda” series and Koizumi the “Super Mario” series. Koizumi was next appointed as Director of Super Mario Sunshine along with Kenta Usui for the Nintendo GameCube and continued as Director and Game Designer of Super Mario Galaxy before moving into a Producer role for the series.

In 2003, Nintendo decided to open a development studio in Tokyo to find new employees that were not interested in relocating to Kyoto where they are headquartered. Nintendo needed someone from their Kyoto office who had experience with game development to head up and lead their new Tokyo studio. They entrusted Yoshiaki Koizumi and Takao Shimizu with this task and sent them to Tokyo to head up Nintendo Entertainment Analysis & Development (EAD) Tokyo.

The studio’s first project was Donkey Kong Jungle Beat for the GameCube, a game that implemented a unique Bongo style controller to control Donkey Kong in a platforming game. The interesting control interface was well received and was a hit with critics, though the game did not achieve the success they were hoping for. The team’s next project was to tackle the successor to Super Mario Sunshine for Nintendo’s upcoming home console, the Nintendo Wii.

Koizumi led the studio as the Director and Game Designer for “Super Mario Galaxy”. The game became an enormous hit for Nintendo and is one of the highest rated titles of all time. Following the completion of “Super Mario Galaxy” Koizumi was promoted to Producer for the “Super Mario” series and Manager of Tokyo Software Development Group No. 2 within Nintendo EAD.

Continuing as the Producer of the “Super Mario” series, Koizumi would oversee the creation of Super Mario Galaxy 2, Super Mario 3D Land, Super Mario 3D World and Super Mario Odyssey. While overseeing the direction of the “Super Mario” series, Koizumi continued to advance within Nintendo. In 2013, he was appointed as a Director of 1-UP Studio, a Nintendo subsidiary. In September 2015, he became Deputy General Manager of the newly formed Entertainment Planning & Development (EPD) Division when Nintendo merged their Entertainment Analysis & Development (EAD) and Software Planning & Development (SPD) divisions.

Also serving as the General Producer for the Nintendo Switch, Koizumi was part of the live streamed Nintendo Switch Presentation where he struck a chord with the Nintendo fanbase. Since the live stream presentation, Koizumi has succeeded Satoru Iwata as the international host of the Nintendo Direct video presentations and started to enter into the limelight with video game fans.